Measurement and visualization of air and fluid movement is useful in such areas as aeronautical engineering, combustion research, and ballistics. Various techniques have been proposed for measuring and visualizing fluid motion, and they can be based on computation of optical flow in a video using an assumption of brightness constancy. Previously proposed techniques have included sound tomography, Doppler LIDAR, and schlieren photography.
Two general techniques to visualize and measure fluid flow include introduction of a foreign substance and optical techniques. In foreign-substance techniques, substances such as a dye, smoke, or particles are introduced into a fluid and then tracked to deduce fluid motion. Optical techniques can include schlieren photography, for example, in which an optical setup is used to amplify deflections of light rays due to refraction to make the fluid flow visible. An alternative to standard schlieren photography is background-oriented schlieren photography (or synthetic schlieren photography), in which fluid flow between each frame of a video and an undistorted reference frame is computed in an attempt to recover fluid motion.